<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>

	<body>
		<canvas id="canvas" style="border: 1px solid black;"></canvas>
		<button id="button">清除画面</button>
		<script>
			window.onload = function() {
				var canvas = document.getElementById("canvas");
				var cxt = canvas.getContext("2d");
				canvas.width = 400;
				canvas.height = 300;
				var canvasx = canvas.getBoundingClientRect().left;
				var canvasy = canvas.getBoundingClientRect().top;
				console.log(canvasx);
				console.log(canvasy);
				canvas.addEventListener("mousedown",downfun);
				canvas.addEventListener("mousemove",movefun);
				canvas.addEventListener("mouseup",upfun);

				var startx;
				var starty;
				var endx;
				var endy;
				cxt.lineWidth = 3;
				var offon = false;
				function downfun(e){
					offon = true;
					startx = e.clientX-canvasx;
					starty = e.clientY-canvasy;
				}
				function movefun(e){
					if(offon == true){
					cxt.beginPath()
					endx = e.clientX-canvasx;
					endy = e.clientY-canvasy;
					cxt.moveTo(startx,starty);
					cxt.lineTo(endx,endy);
					cxt.stroke();
					
					startx = endx;
					starty = endy;

					}
				}
				function upfun(e){
					offon = false;
					cxt.closePath();
				}
				document.getElementById("button").onclick = function(){
					cxt.clearRect(0,0,canvas.width,canvas.height);
				}

			}
		</script>
	</body>

</html>